In recent years, development of organic electro luminescent (organic EL) devices, which emit light by themselves using electrons and positive holes as carriers, has been progressing rapidly. Organic EL devices are characteristically superior to liquid crystal devices, which do not emit light by themselves and therefore need a backlight, in terms of possibility of thickness and weight reduction, visibility, and the like.
An organic EL device generally includes a pair of facing substrates having an electrode on their facing sides and an emitting layer interposed between the substrates. The emitting layer is formed of an organic thin film containing a light-emitting substance that emits light upon voltage application. The light emission mechanism of the organic. EL device is as follows; a voltage is applied to the organic thin film between the positive and the negative electrode to inject holes and electrons; the holes and electrons are recombined in the organic thin film; and the excitons generated by the recombination return to the ground state thereupon to emit light.
An organic EL device should have, in addition to the emitting layer, a hole or an electron injection layer for increasing the efficiency of hole or electron injection and a hole or an electron transport layer for increasing the hole/electron recombination efficiency, hens the emitting layer and each electrode. Consequently, an organic EL device has a complicate multi-layer structure and involves many manufacturing steps. Moreover, there are many restrictions in selecting the electrode materials forming positive and negative electrodes because the work function should be taken into consideration.
Light-emitting electrochemical cells (LECs) are recently attracting attention as a self-light-emitting device that can cope with the above described problems. An LEC generally has an emitting layer containing a salt and an organic light-emitting substance. With a voltage applied, the cations and anions of the salt move in the emitting layer toward the negative and the positive electrode, respectively, to provide a large electric field gradient (electric double layer) on the electrode interface. Because the thus formed electric double layer facilitates electron and hole injection by the respective electrodes, an LEC does not need to have such a multi-layer structure as needed by organic EL devices. Furthermore, in the manufacture of LECs, because consideration of the work function is not required in selecting the electrode materials, limitation to the electrode materials is not so great. For these reasons, LECs are promising self-emitting devices with expectation of great cost reduction compared with organic EL devices.
The salt used in the LECs is often an ionic compound, including inorganic ones, such as lithium salts and potassium salts, and organic ones, such as an ionic liquid (see, e.g., Patent Literatures 1 to 4 listed below). One of the merits of using an ionic compound consists in that cations and anions are easily re-arranged on the electrode interfaces to easily form an electric double layer, thereby facilitating injection of holes and electrons.
In particular, an emitting layer containing an organic ionic compound, such as an ionic liquid, has been under study as in Patent Literatures 1 and 2 because the use of the organic ionic compound increases the rate of re-arrangement of ions on the electrode interfaces. Patent Literature 5 below teaches increasing the life and efficiency of organic light-emitting electrochemical devices, including LECs, by the use of a non-polymeric organic ionic compound containing one ion having a functional organic group and another ion that is so small as to act as a mobile ion in films containing the organic ionic compound. The non-polymeric organic ionic compound is described as being composed of a mono-charged organic cationic compound and a mono-charged anionic compound. Non-Patent Literature 1 proposes a compound composed of an imidazolium cation and a sulfuric ester anion for use as an ionic compound in LECs.